1. Field of the Invention
The present invention relates to a ceramic capacitor excellent in various characteristics required for a capacitor.
2. Description of the Related Art
A capacitor composed of a capacitor element in which a dielectric ceramic composition is interposed between electrodes has been conventionally, widely known, and a large number of dielectric materials which can be suitably used in such a capacitor have been developed. The electrical characteristics required for these dielectric materials are, for example, a large dielectric constant, a small temperature coefficient of dielectric constant, a small dielectric loss, a small DC bias voltage voltage electric field dependence of the dielectric constant, a small AC bias voltage voltage electric field dependence of the dielectric loss, and a large insulation resistance. In situations where the capacitor is used in an electronic circuit, it is sometimes required to particularly, stably maintain a capacitance over a wide temperature range. Such a temperature coefficient of a capacitance (to be abbreviated a T.C.C. hereinafter) is defined as, for example, a rate of change of .+-.10% or less in a capacitance over a temperature range of -25.degree. C. to 85.degree. C. in accordance with the B specification of the EIAJ (Electronic Industries Association of Japan) specifications, and that of .+-.20% or less over the same temperature range in accordance with their C specification; and a rate of change of .+-.15% or less in a capacitance over a temperature range of -55.degree. C. to 125.degree. C. in accordance with the X7R specification of the EIA (Electronic Industries Association) specifications, that of .+-.22% or less over the same temperature range in accordance with their X7S specification, and that of -33% to +22% over the same temperature range in accordance with their X7T specification.
In the case of an element of a stacked type, since electrode layers and a dielectric layer are sintered integrally, it is necessary to use an electrode material which is stable even at the sintering temperature of a dielectric material. Therefore, if the sintering temperature of the dielectric material is high, an expensive material such as platinum (Pt) or palladium (Pd) must be used. For this reason, it is required that sintering at a low temperature of, e.g., about 1,150.degree. C. (or less) is possible so that an inexpensive material such as Ag can be used.
An example of a conventionally known dielectric ceramic composition is a solid solution of, e.g., a stannate, zirconate, or titanate in barium titanate (BaTiO.sub.3) as a base material.
The sintering temperature, however, of the BaTiO.sub.3 -based material is as high as 1,300.degree. C. to 1,400.degree. C. Therefore, an expensive material, such as Pt or Pd, which can resist high temperatures, must be inevitably used as the electrode material, and this results in a high cost.
In order to solve this problem with the BaTiO.sub.3 -based material, studies have been made on various types of compositions. Examples of a composition mainly consisting of lead iron niobate (Published Unexamined Japanese Patent Application No. 57-57204), a composition mainly consisting of lead magnesium niobate (Published Unexamined Japanese Patent Application No. 55-51759), a composition mainly consisting of lead magnesium tungstate (Published Unexamined Japanese Patent Application No. 55-144609), and a composition mainly consisting of lead magnesium iron tungstate (Published Unexamined Japanese Patent Application No. 58-217462).
No dielectric ceramic composition, however, has been obtained yet which has a high dielectric constant and a small change in dielectric constant with temperature changes over a wide temperature range of, e.g., -55.degree. C. to 125.degree. C., which is excellent in electrical characteristics such as an insulation resistance and a breakdown voltage, and which can be sintered at a low temperature.
Independently of these studies, another study has been made to obtain a ceramic composition having a good temperature coefficient by mixing compositions different in the temperature coefficient of dielectric constant. As an example, Published Unexamined Japanese Patent Application No. 59-203759 discloses mixed sintering of a lead composite perovskite material (to be referred to as a relaxor hereinafter). However, that material has a large T.C.C. and is therefore insufficient in temperature coefficient.
As a dielectric material which is excellent in all of the electrical characteristics described above and has extremely good temperature coefficient, there is conventionally provided a dielectric composition obtained by sintering a mixture of a calcined powder of a lead composition perovskite material with a calcined powder of a BaTiO.sub.3 -based material. For example, Published Unexamined Japanese Patent Application No. 61-250904 discloses a technique by which a dielectric ceramic composition having good temperature coefficient is obtained by sintering a mixture of a calcined powder of a Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 -based material and a calcined powder of a BaTiO.sub.3 -based material.
In the BaTiO.sub.3 -based material or the composite form of the relaxor and the BaTiO.sub.3 -based material, however, the AC bias voltage dependence of the dielectric loss (tan.delta.) of BaTiO.sub.3 is large. Therefore, when the thickness of a dielectric layer of a capacitor, particularly a multilayered ceramic capacitor (to be abbreviated as an MLC hereinafter) is decreased, the results are not only a reduction in dielectric constant but also a large increase in dielectric loss caused by an increase in AC bias voltage applied per layer. Consequently, it is impossible to satisfy tan.delta..ltoreq.2.5% (when evaluated at 1 Vrms as a measurement voltage) which is the B specification of EIAJ and the X7R specification of EIA. Therefore, the large bias voltage dependence of the dielectric loss of the BaTiO.sub.3 -based material is a serious problem in miniaturizing the MLC, i.e., decreasing the thickness of a dielectric layer.
In addition, the BaTiO.sub.3 -based material has a large rate (called an aging rate) at which the dielectric constant decreases with time. Therefore, when a capacitor fabricated using this material is used for extended periods, no desired capacitance can be obtained. For example, although Published Unexamined Japanese Patent Application No. 57-62521 discloses a capacitor which satisfies the X7R specification, the aging rate of this capacitor is as very large as 3% or more.
The BaTiO.sub.3 -based material also has which the dielectric constant decreases upon application of a DC bias voltage. For this reason, if, for example, a voltage of 25 V is applied to an MLC fabricated using this material and having a dielectric layer with a thickness of 12.5 .mu.m, the capacitance of the MLC undesirably decreases by 20% or more.
The relaxor, on the other hand, has a problem of a breakdown voltage lower than that of the BaTiO.sub.3 -based material. Since it is assumed that the thickness of a dielectric layer of a capacitor will be further decreased in future, it is necessary to use a relaxor having a higher breakdown voltage in order to fabricate a capacitor.
Furthermore, when the above conventional dielectric material is used to fabricate an MLC while an inexpensive material mainly consisting of Ag is used as electrode layers, the Ag diffuses into the dielectric layer upon integral sintering of the electrode layers with the dielectric layer. This results in degradation in reliability such as a humidity load resistance or a reduction in a breakdown voltage. In addition, if the dielectric layer of the fabricated MLC is thin, the above Ag diffusion leads to inconveniences such as a reduction in the insulation resistance of the dielectric layer and a short circuit of the electrodes. This consequently makes it difficult to decrease the thickness of the dielectric layer, that is, to miniaturize the MLC.